As conventional static pressure type non-contact gas seals, a static pressure type non-contact gas seal configured as below is known (for example, refer to Patent Document 1). That is, the static pressure type non-contact gas seal includes a tubular seal case; a rotary sealing ring fixed to a rotary member; a stationary sealing ring held in an inner peripheral surface of the seal case so as to be axially movable in a state in which it faces the rotary sealing ring; an opening force generating means that supplies seal gas to the space between sealing end surfaces as facing end surfaces of both the sealing rings, from a joined seal gas passage passing through the seal case and the stationary sealing ring, to thereby generate an opening force acting on the stationary sealing ring in a direction in which the space between the sealing end surfaces is opened; and a closing force generating means that generates a closing force acting on the stationary sealing ring in a direction in which the space between the sealing end surfaces is closed with a biasing force generated by a spring interposed between the seal case and stationary sealing ring. The balance of an opening force acting on the stationary sealing ring in which the space between the sealing end surfaces is opened, an opening force generated by the opening force generating means, and a closing force generated by the closing force generating means is maintained by a pressure of a sealed fluid acting on a portion of an outer peripheral surface of the stationary sealing ring that faces a sealed fluid region excluding the sealed end surfaces so that the sealing end surfaces are kept in a mutual non-contact state.
Specifically, in the static pressure type non-contact gas seal (hereinafter referred to as “conventional gas seal”) having such a configuration, the sealing end surfaces are kept in a mutual non-contact state in which an opening force (an opening force resulting from a pressure (static pressure) of the seal gas introduced between the sealing end surfaces (according to the shape, etc. of the stationary sealing, further including an opening force resulting from a pressure of the sealed fluid acting on the portion of the outer peripheral surface of the stationary sealing ring that faces the sealed fluid region excluding the sealing end surfaces)) acting on the stationary sealing ring in a direction in which the space between the sealing end surfaces is opened is balanced with a closing force (a closing force generated by a spring that presses and biases the stationary sealing ring toward the rotary sealing ring) acting on the stationary sealing ring in a direction in which the space between the sealing end surfaces is closed. Here, the pressure of the seal gas introduced into sealing end surfaces is set to be higher than the pressure of the sealed fluid region so as to vary according to this pressure, and the spring force (spring load) of the spring that determines the closing force is set according to the pressure of the seal gas so that the gap between the sealing end surfaces is set appropriately (generally, 5 to 15 μm).
Thus, similar to a dynamic pressure type non-contact gas seal in which the sealing end surfaces are kept in a mutual non-contact state by generating a dynamic pressure resulting from the sealed fluid in the space between the sealing end surfaces, the conventional gas seal can keep the sealing end surfaces in a mutual non-contact state to seal the sealed fluid well for a prolonged period of time without causing seizing of the sealing end surfaces. Moreover, the conventional gas seal can also satisfactorily seal the gases, which could not be sealed by the dynamic pressure type non-contact gas seal, and thus it can has wide applications as compared with the dynamic pressure type non-contact gas seal. Specifically, the dynamic pressure type non-contact gas seal, as widely known, is configured such that a dynamic pressure generating groove is formed in one of sealing end surfaces that rotates relative to each other, and a dynamic pressure resulting from a sealed fluid is generated between the sealing end surfaces by the action of the dynamic pressure generating groove, thereby keeping the sealing end surfaces in a mutual non-contact state. Basically, the dynamic pressure type non-contact gas seal allows the sealed fluid to leaks to the outside of an apparatus from the space between sealing end surfaces. Accordingly, in a case in which the sealed fluid is a fluid, such as a toxic gas, an inflammable gas, or an explosive gas, having the property that does not allows leaking to the outside, the dynamic pressure type non-contact gas seal cannot be used. In contrast, in the conventional gas seal that is a static pressure type non-contact gas seal, the seal gas is jetted toward the sealed fluid region (and non-sealed fluid region) from the space between the sealing end surfaces. Therefore, even in rotary apparatuses which deal with gases, such as toxic gases, flammable gases, and explosive gases, which do not allow leaking, the conventional gas seal can be suitably used.
Patent Document 1: Japanese Patent Application Publication No. 2000-329238 (FIG. 1)
However, in the conventional gas seal, if supply of the seal gas to the space between the sealing end surfaces has stopped due to certain factors (failure, erroneous operation, and the like of a seal gas supply system) during operation (or during driving of a rotary shaft) of a rotary apparatus equipped with the gas seal, the following problems will occur.
That is, in the conventional gas seal, since the spring is used as the closing force generating means, a closing force generated by the spring acts at all times. Accordingly, as described above, if supply of the seal gas has stopped unexpectedly and thereby a closing force resulting from the seal gas vanishes, the stationary sealing ring is suddenly moved to the rotary sealing ring by the biasing force (closing force) of the spring, and collides severely against the rotary sealing ring. As a result, there is a possibility that the sealing end surfaces may be damaged or destructed. This problem becomes remarkable, particularly under high-pressure conditions in which the pressure of the sealed fluid is high. Specifically, under the high-pressure conditions, the pressure of the seal gas is required to be set higher according to the pressure of the sealed fluid. Therefore, the load of the spring to be balanced with an opening force resulting from the seal gas is also compelled to be increased. Accordingly, when supply of the seal gas has stopped, the stationary sealing ring collides extremely severely against the rotary sealing ring, and therefore the damage or destruction of the sealing end surfaces is increased.
The invention has been finalized in consideration of the above-described problems, and it is the object of the invention to provide a static pressure type non-contact gas seal which, even when supply of seal gas to the space between sealing end surfaces has stopped unexpectedly, can be safely used with no possibility that the sealing end surfaces collide severely against each other and consequently the sealing end surfaces are damaged or destructed.